Internet Protocol (IP) Multimedia Subsystem is a standardized set of specifications that describes a Next Generation Network (NGN) with a generic architecture for Internet Protocol (IP)-based telephony and multimedia services. Third Generation Partnership Project (3GPP) and Third Generation Partnership Project 2 (3GPP2) enable and support the evolution of mobile networks beyond second-generation (2G) mobile systems such as Global System for Mobile Communications (GSM) and cdma2000. IP Multimedia Subsystem (IMS) was also adopted by other standardization bodies (e.g., Wi-MAX and TISPAN) for their future networks. IMS uses Session Initiation Protocol (SIP) to control multimedia communication sessions such as video and Voice calls over IP (VoIP), Instant Messaging, and presence. Some of the benefits of IMS core networks include, for example, support for VoIP and multimedia services based on standardized interfaces and reusable components, network optimization, policy control, and charging. Telecommunication Service Providers (TSPs) are investing in and deploying IMS/IP core networks to provide these benefits to their subscribers.
According to the approach defined in 3GPP specifications, all services are anchored in the IMS domain as defined in Single Radio Voice Call Continuity (SR-VCC) 3GPP work item (for more information, please refer to 3GPP TS 23.216, 3GPP TS 24.206, 3GPP TS 24.216, and 3GPP TS 24.237). However, TSPs with a legacy mobile network typically want to leverage past investments in already deployed legacy network circuit-switched services. Such services include, for example, basic voice and supplementary services, voicemail, directory services, Intelligent Network (IN)-based services (e.g., prepaid, local number portability, caller tune), regulatory features (e.g., Lawful intercept, e911), Short Message Service (SMS), Multimedia Messaging Service (MMS), Unstructured Supplementary Service Data (USSD), and so on. However, IMS core network subscribers do not generally have voice service parity with legacy networks without deploying Telephony Application Servers (a/k/a MMTel servers) in the IMS domain. One reason for this is that the stable and customized natures of voice-related services in legacy domains generally limit the ease with which such services can be migrated to SIP-based application servers.
3GPP and 3GPP2 networks predominantly deliver legacy advanced services (e.g., LNP, CNAM, pre-paid, caller tune) via Service Control Points (SCP) connected via Intelligent Network (IN) using protocols such as Customized Application for Mobile Network Enhanced Logic (CAMEL) (if in 3GPP networks). Although the 3GPP specification proposes use of an IMS Service-Switching Function (IM-SSF) as an adaptation entity between IMS and the SCP to provide legacy supplementary services and other service features including Value-Added Services (VAS) to IMS subscribers, the proposed implementation is substantially limited. For example, because the signaling protocols between IMS (SIP) and the SCP (CAMEL application part (CAP)) are substantially different in terms of signaling messages and state model, complex mapping and state model implementations in the IM-SSF generally are needed for each supported legacy service. Moreover, since only a subset of CAP is utilized for communications between the SCP and the IM-SSF, many CAP to SIP conversions are typically restricted in scope. As a result, much of the legacy service features in the legacy environment is not available to IMS core network subscribers through the IM-SSF.